收费全文 | 1388篇 |
免费 | 41篇 |
林业 | 72篇 |
农学 | 40篇 |
基础科学 | 14篇 |
129篇 | |
综合类 | 237篇 |
农作物 | 92篇 |
水产渔业 | 58篇 |
畜牧兽医 | 676篇 |
园艺 | 12篇 |
植物保护 | 99篇 |
2021年 | 16篇 |
2020年 | 15篇 |
2019年 | 14篇 |
2018年 | 11篇 |
2017年 | 15篇 |
2016年 | 18篇 |
2015年 | 19篇 |
2014年 | 19篇 |
2013年 | 57篇 |
2012年 | 48篇 |
2011年 | 53篇 |
2010年 | 46篇 |
2009年 | 23篇 |
2008年 | 51篇 |
2007年 | 46篇 |
2006年 | 47篇 |
2005年 | 57篇 |
2004年 | 54篇 |
2003年 | 47篇 |
2002年 | 35篇 |
2001年 | 34篇 |
2000年 | 39篇 |
1999年 | 25篇 |
1998年 | 12篇 |
1997年 | 11篇 |
1996年 | 14篇 |
1995年 | 18篇 |
1992年 | 19篇 |
1991年 | 24篇 |
1990年 | 22篇 |
1989年 | 23篇 |
1988年 | 20篇 |
1987年 | 18篇 |
1986年 | 22篇 |
1985年 | 25篇 |
1984年 | 19篇 |
1983年 | 15篇 |
1982年 | 18篇 |
1981年 | 11篇 |
1979年 | 14篇 |
1977年 | 16篇 |
1976年 | 18篇 |
1975年 | 13篇 |
1974年 | 14篇 |
1973年 | 23篇 |
1972年 | 20篇 |
1971年 | 15篇 |
1966年 | 10篇 |
1948年 | 12篇 |
1947年 | 13篇 |
- 1. This paper provides a detailed review of the status and distribution of wintering seaducks (Scaup, Eider, Long-tailed Duck, Common Scoter, Velvet Scoter and Red-breasted Merganser) in Britain and Ireland. It is based on a literature review plus analysis of National Waterfowl Counts (NWC) and other data. A summary of current knowledge of wintering seaducks is provided together with information on threats to populations, conservation goals and research needs.
- 2. Each species is examined in turn, with the emphasis on aspects of the population, habitat use, current distribution, trends and history. Important areas for individual species are highlighted.
- 3. Overall, seaduck populations have changed considerably, those for Eider and Red-breasted Merganser having expanded greatly, whilst those for Scaup, Common and Velvet Scoter have declined during historical and/or recent times. The trend for Long-tailed Ducks is unclear. British population estimates are considered to be out of date and need to be revised. The premier areas for seaduck communities are highlighted, thus providing basic information for conservation action and future monitoring.
- 4. Seaduck populations are threatened in a number of ways and the main factors are discussed. Oil pollution and the actions of commercial fisheries are the most important current threats. Conservation goals are suggested. These include an urgent need to develop appropriate domestic legislation to facilitate the protection of seaduck populations. Such legislation is currently almost entirely lacking.
- 5. Some priorities for research, particularly for better population monitoring of seaducks in British and Irish waters, are provided. These include: alterations to the NWC scheme to allow the collation of opportunistic counts, made when the conditions are amenable for censusing seaducks; extension of the scheme into areas not currently surveyed (e.g. most parts of Ireland); and the establishment of intensive survey programmes to monitor seaducks in the key areas of Britain and Ireland. It is hoped that this paper will serve to stimulate interest and commitment from all who play a part in safeguarding our seaduck heritage.
Although litter decomposition and nutrient release patterns have been studied in cocoa agroforestry systems in general, studies focusing on organic and conventional cocoa systems are lacking which is critical as organic farms are particularly dependent on nutrient returns from decomposing litter.
Materials and methodsDynamics in leaf litter decomposition and the mineralisation of macro- and micro-nutrients in organic and conventional cocoa agroforestry systems were studied using the litterbag technique for 12 months.
ResultsThe average monthly mass loss was more than two times higher on organic farms (9.2–14.4 g month?1) compared to conventional farms (4.2–7.3 g month?1) in the first five months. The annual rate of decomposition (k) was higher on organic farms (1.9) compared to conventional systems (1.4). The time required for 50% (t50) and 99% (t99) decomposition of leaf litter was both lower on organic farms (t50?=?0.4 years, t99?=?2.6 years) than conventional farms (t50?=?0.5 years, t99?=?3.5 years). The estimated k values for macro- and micro-nutrients on organic cocoa systems ranged from 2.3 for calcium to 4.5 for potassium compared to 1.6 (Ca) to 2.8 (K) on conventional farms. The k values of all nutrients (except nitrogen and phosphorus) were significantly greater on organic farms than conventional systems. The estimated k values for both litter decomposition and nutrient mineralisation correlated with soil pH and moisture content, but not initial litter chemistry.
ConclusionsOrganic management of smallholder cocoa agroforestry systems enhanced leaf litter decomposition and nutrient mineralisation through improved soil conditions. Thus, organic management of cocoa agroforestry systems may contribute to sustainable cocoa production in smallholder systems through enhanced nutrient return from litter decomposition.
相似文献